1. Field of the Invention
The invention relates to an optical fiber amplifier, an optical fiber laser, and an optical fiber for optical amplifier (OFA) applicable thereto as an optical amplification medium.
2. Related Background Art
An optical fiber for optical amplifier (OFA) has a function of amplifying signals propagating therethrough by pumping light supplied thereto, and is an optical device applicable as an optical amplification medium in an optical fiber amplifier, an optical fiber laser, and the like. In general, an OFA is a silica-based optical fiber and has a core region doped with a luminescent element for signal amplification. As this luminescent element, for example, a rare earth element, preferably Er can be used. An OFA to which Er, as a luminescent element, is added can amplify signals of a wavelength band of 1.55 xcexcm when pumping light of a wavelength of 1.48 xcexcm or 0.98 xcexcm is supplied thereto. In such an OFA, increasing a signal gain is important, and some kinds of techniques for meeting this requirement have been proposed.
For example, an OFA disclosed as a conventional technology in Japanese Patent Laid-Open No. 135548/1998 (Document 1) has a core region having a refractive index of n1, an inner cladding region provided on the periphery of the core region and having a refractive index n2 ( less than n1), and an outer cladding region provided on the periphery of the inner cladding region and having a refractive index n3 ( less than n2). The core region is doped with a luminescent element. This optical fiber makes the pumping light propagate as light of an inner cladding mode (a mode in which the pumping light propagates while being confined in both the core region and inner cladding region), and makes signals as light of a core mode (a mode in which the signals propagate while being supported in the core region). Such an OFA will hereinafter be referred to as a cladding-pumped fiber.
Japanese Patent Laid-Open No 238883/1991 (Document 2) discloses a structure suitable for above-mentioned cladding-pumped fiber. It is described that, in this cladding-pumped fiber, pumping light can propagated as an inner cladding mode light, so that a coupling efficiency of the pumping light in the fiber is high. Further, since the pumping light also propagates through an inner cladding region containing no luminescent element and a transmission loss of the pumping light is small, a gain efficiency can be improved.
A cladding-pumped fiber disclosed in International Laid-open Publication PCT/EP96/04187 (Document 3) works to induce a mode coupling between an inner cladding mode and a core mode with respect to pumping light by employing a non-circular cross-sectional shape to the inner cladding region. Concerning the pumping light introduced into this cladding-pumped fiber, the inner cladding mode is more effectively coupled to the core modes, and a gain efficiency is thereby increased to a high level.
Further, in a cladding-pumped fiber disclosed in Japanese Patent Laid-Open No. 107345/1998 (Document 4), signals propagate with not only light of a fundamental mode (core mode) but also that of a high-order mode while being confined in a core region, and a signal gain is thereby increased only light of the fundamental mode is outputted to outside by a grating cutting of that of the high-order mode.
The inventors discussed the above-mentioned prior techniques to discover the following problems. Namely, in the cladding-pumped fiber of Documents 1 and 2, the pumping light of inner cladding mode cannot effectively pump the luminescent element added to the core region, thereby the increasing of a signal gain could not be sufficiently attained. It is difficult to control the manufacturing of a cladding-pumped fiber having a non-circular cross-sectional shape of an inner cladding region, and further the mechanical strength of such optical fiber is liable to decrease.
In the cladding-pumped fiber of Documents 3 and 4 through which signals propagate with not only light of a fundamental mode but also that of a high-order mode while being supported in a core region, it is difficult to completely cut off the high-order mode light by a grating. The leakage high-order light induces a high noise figure. Moreover, a splice loss between this cladding-pumped fiber and another optical fiber becomes large, and the pumping of the high-order mode occurs in the latter optical fiber, this matter also constitutes a noise figure.
The present invention has been accomplished to solve the above-mentioned problems, and an object of the present invention is to provide an OFA having a high gain efficiency, capable of being manufactured easily, having a high mechanical strength, not inducing a large splice loss with respect to any other optical fiber, and not inducing noise in a signal wavelength, an optical fiber amplifier and an optical fiber laser to both of which said OFA can be applied as an optical amplification medium.
An OFA according to the present invention is a silica-based optical fiber having a function of amplifying signals when pumping light is supplied thereto, and comprises a core region having a refractive index n1, an inner cladding region provided on the periphery of the core region and having a refractive index n2( less than n1), and an outer cladding region provided on the periphery of the inner cladding region and having a refractive index n3( less than n2). The core region has a structure ensuring a single mode with respect to the signals, and the inner cladding region has a structure ensuring a multi-mode with respect to the pumping light. A signal amplifying luminescent element is aided to at least a part of the core region.
In particular, the OFA according to the present invention comprises one or more mode coupling gratings, each allowing core mode light to pass therethrough and inducing a mode coupling between an inner cladding mode and a core mode with respect to the pumping light.
In the OFA, the pumping light initially introduced thereinto propagates as light of an inner cladding mode, and the inner cladding mode is then converted into a core mode by a mode coupling effect of the mode coupling grating. The pumping light of the core mode pumps the luminescent element added to at least the part of the core region of the OFA. On the other hand, signals propagate as light of the core mode and pass through the mode coupling grating while remaining its mode. Consequently, amplified signals are obtained from the OFA.
At least one of mode coupling gratings in the OFA according to the present invention may include a long-period grating which induces a mode coupling between an inner cladding mode and a core mode whose propagation direction is identical with that of the inner cladding mode. In this case, the period of the long-period grating is preferably 10 xcexcm-400 xcexcm. When the pumping light reaches a long-period grating, the inner cladding mode is converted into a core mode whose propagation direction is identical with that of the inner cladding mode by the mode coupling effect of the mode coupling grating.
At least one of mode coupling gratings in the OFA according to the present invention may include a tilted fiber Bragg grating which induces a mode coupling between an inner cladding mode and a core mode whose propagation direction is contrary to that of the inner cladding mode. In this case, an angle of inclination of the tilted fiber Bragg grating with respect to a predetermined axis, for example, an optical axis is preferably 0.5xc2x0-20xc2x0. When the pumping light reaches the tilted fiber Bragg grating, the inner cladding mode is converted into a core mode whose propagation direction is contrary to that of the inner cladding node by the mode coupling effect of this tilted fiber Bragg grating.
As clearly disclosed in U.S. Pat. No. 5,703,978, a long-period grating is a grating inducing a mode coupling between a core mode and a cladding mode propagating through an optical fiber, and clearly distinguished from a fiber Bragg grating which reflects light of around a predetermined wavelength without inducing a rode coupling. In a long-period grating, a grating period is so set that a phase difference between a core mode and a cladding mode becomes 2xcfx80 in order to obtain strong power conversion from the core mode into the cladding mode. Therefore, since the long-period grating works so as to couple the core mode with the cladding mode, the core mode attenuates over a narrow band around a predetermined wavelength (which will hereinafter be referred to as a loss wavelength).
The optical fiber amplifier according to the present invention comprises an OFA having the above-mentioned mode coupling grating (OFA according to the present invention), and one or more pumping light sources supplying pumping light to the OFA. In accordance with the optical fiber amplifier, a high gain is obtained because the OFA is employed as a signal amplifying optical amplification medium.
When the optical fiber amplifier comprises pumping light sources each supplying pumping light of wavelengths different from each other, it is preferable that the optical fiber for optical amplifier includes fiber Bragg gratings, which selectively reflect light of a predetermined wavelength, arranged so as to sandwich the one or more mode coupling gratings. Each of the fiber Bragg gratings may includes either a Bragg grating reflecting light of a specific wavelength without inducing a mode coupling, or a tilted Bragg grating inclined at a predetermined angle with respect a predetermined axis and having a mode coupling effect.
The optical fiber laser according to the present invention comprises an OFA having the above-mentioned structure (OFA according to the present invention), one or more pumping light sources each supplying pumping light to the OFA, and a resonator resonating signals amplified by the OFA. The resonator in the optical fiber laser may constituted by a Fabry-Perot resonator or a ring type resonator. In accordance with such an optical fiber laser, a high laser oscillation efficiency is obtained by the application of the OFA as an optical amplification medium for amplifying signals (induced emission light). In the optical fiber laser, the pumping light sources may also has structures which supply pumping light of wavelengths different from each other. In this case, it is preferable that the OFA comprises fiber Bragg gratings, selectively reflect light of a predetermined wavelength, arranged so as to sandwich the one or more mode coupling gratings. Each of the fiber Bragg gratings may include either a Bragg grating reflecting light of a specific wavelength without inducing a mode coupling, or a tilted Bragg grating inclined at a predetermined angle with respect to a predetermined axis and having a mode coupling effect.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.